break out in laughter

break out in hives
じんましんが出る

break out in laughter
ドッと笑いだす

break out into a heavy sweat
突然大量の汗が噴き出る

break out into a huge smile
（突然）大きな笑みを浮かべる

She noticed an old friend walking toward her, and broke out into a huge smile. 
彼女は旧友が近づいてくるのに気付くと、大きな笑みを浮かべた。